Photolithography for Making Electrochemical Measurements

ABSTRACT

An apparatus for electrochemical experimentation with an isolated microstructural region on a surface comprising a metal sample coated with a photoresist, a region of interest, a light source, comprising optoelectronic devices such as spatial light modulators or digital micromirror devices for direct modulation of the light distribution itself and avoiding the use of a mask, wherein the exposed region is created by light from the light source and wherein the metal sample is immersed. A method for isolating microstructural regions or features on a surface for electrochemical experimentation comprising the steps of providing a metal sample, coating the metal sample, selecting a region of interest, creating exposed photoresist with direct modulation of the light distribution itself by optoelectronic devices such as spatial light modulators or digital micromirror devices and without a mask.

This application claims priority to and benefit of U.S. PatentApplication No. 61/540,093 filed Sep. 28, 2011 and U.S. patentapplication Ser. No. 13/618,214 filed on Sep. 14, 2012 and U.S. patentapplication Ser. No. 14/988,575 filed on Jan. 5, 2016, the entireties ofeach are herein incorporated by reference.

BACKGROUND

This procedure provides a technique for conducting electrochemicalexperiments on precise microstructural features on a material surface.This procedure furthermore minimizes loss of signal due to high solutionresistance.

Earlier procedures suffer from several disadvantages. Examples includemicrocapillaries and lacquers. Microcapillaries have the disadvantagesof high solution resistance through the capillary, solution leakage atthe seal with the sample surface, and imprecise capillary placement.

Using lacquers is a technique that was suggested in the 1970s andconsisted of coating the surface to be sampled with a lacquer and thento make small pinholes over the regions to be investigated. Again, theuse of lacquers as a technique suffers from several disadvantages.

One of the challenges associated with electrochemical testing is that itis difficult to determine individual contributions to a measured currentespecially when the sample area comprises multiple grains, grainboundaries, precipitates, etc. The heterogeneity of such areas result incompeting kinetic processes that contribute to the overall current.

For studies that are aimed at determination for example, of corrosionresistance or catalytic activity, it would be beneficial to have aversatile technique that can isolate areas of interest.

Localized experimental procedures using micro-capillaries (1-3) to probesmall areas of the sample surfaces have been developed previously.Though the micro-capillary technique has been in widespread use for thepast 15 years, aspects of the technique make it undesirable for certainexperimental procedures. For example, the relatively fastpotentiodynamic sweep rates required to prevent cell leakage or tipblockage of the micro-capillary prevent scanning at rates as slow as 10mV/min and the micro-capillary tip diameter can affect the limitingcurrent passing through the cell. In addition a flat, polished surfaceis needed for this technique.

Here, a technique for making electrochemical measurements on isolatedindividual phase regions of known crystalline orientation in a duplexstainless steel is demonstrated. An ultraviolet-sensitive photoresist isused to mask the excluded portions of the sample and a 355 nm laserexposes only portions of the ferrite matrix or cross-sections ofaustenite dendrites. Initial impedance measurements indicate arelatively low solution resistance in seawater and the polarizationscans of the ferrite and austenite phases were consistent with bulkpolarization measurements.

BRIEF SUMMARY OF THE INVENTION

This procedure provides a technique for conducting electrochemicalexperiments on precise microstructural features on a material surface.

This procedure furthermore minimizes loss of signal due to high solutionresistance.

This disclosure describes and demonstrates the utility and viability ofa novel experimental technique, Selective Masking by Photolithography(SMP), for making electrochemical measurements on individual phase orisolated regions of an alloy. Here, a technique for makingelectrochemical measurements on isolated individual phase regions ofknown crystalline orientation in a duplex stainless steel isdemonstrated. An ultraviolet-sensitive photoresist is used to mask theexcluded portions of the sample and a 355 nm laser exposes only portionsof the ferrite matrix or cross-sections of austenite dendrites.

In this disclosure, the technique was used to isolate individual ferriteand austenite phases from their neighbors on a polished duplex stainlesssteel sample (alloy 2205). Alloy 2205, UNS S32205, is a corrosionresistant alloy that consists of approximately equal amounts ofδ-ferrite and γ-austenite phases.

Polarization scans, electrochemical impedance, and critical pittingtemperature experiments were then performed on these isolated regions.

This novel technique for making electrochemical measurements onindividual phase or isolated regions of a metal or alloy is disclosed.The technique, called Selective Masking by Photolithography (SMP), usesa hardened photoresist coating to mask the excluded portions of thesample and 355 nm laser pulses are employed to expose individual grainsor regions of interest. The size of the exposed area can range from tensof microns to millimeters. Localized electrochemical DC and ACmeasurements and critical pitting temperature determinations for the twophases in a duplex stainless steel were used to show the utility andviability of SMP.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the procedure used to expose andisolate individual phases and the corresponding images of each step fora ferrite region of a 2205 steel sample. A green (UV blocking) filter isused in b) and c) to prevent light emission from the microscopeilluminator from further exposing the photoresist.

FIG. 2 illustrates two views “a” and “b”. “a” is a schematic diagramshowing the set-up for providing a conducting path for the electrons toand from the active surface and how the set-up was electrically shieldedfrom the electrolyte. “b” is an optical microscopy image showing thelayers of waterproof material used to electrically isolate a specificwindow in order to ensure only the properties of the phase type ofinterest were measured.

FIG. 3 is an optical microscopy image of an austenite region of interestthat will be uncovered via ultraviolet exposure of the insulatingphotoresist.

DETAILED DESCRIPTION

This procedure provides a technique for conducting electrochemicalexperiments on precise microstructural features on a material surface.

This procedure furthermore minimizes loss of signal due to high solutionresistance.

This disclosure describes and demonstrates the utility and viability ofa novel experimental technique, Selective Masking by Photolithography(SMP), for making electrochemical measurements on individual phase orisolated regions of an alloy.

In this disclosure, the technique was used to isolate individual ferriteand austenite phases from their neighbors on a polished duplex stainlesssteel sample (alloy 2205). Alloy 2205, UNS S32205, is a corrosionresistant alloy that consists of approximately equal amounts ofδ-ferrite and γ-austenite phases.

Polarization scans, electrochemical impedance, and critical pittingtemperature experiments were then performed on these isolated regions.In this work, the sample surfaces were flat and polished but thetechnique could be used to study irregular surfaces.

Although the application for this work is related to corrosion, SMP haswide applicability to any electrochemical study that explores thebehavior of individual phase or isolated regions of interest.

Example 1

Sample Preparation was conducted as follows.

Duplex stainless-steel 2205 cast samples were provided by Wärtsilä. Thephase compositions were determined via the energy dispersive x-rayspectroscopy (EDS) probe on a LEO 1550 Scanning Electron Microscope(SEM). The measured compositions are shown in Table 1.

TABLE 1 Composition (atomic %) of the δ and γ phases in the 2205samples-as determined using EDS. Phase Fe Cr Ni Mo Mn Si C γ-Austenite66.48 22.16 6.71 1.43 1.24 1.98 0.18 δ-Ferrite 63.95 23.89 4.38 3.830.84 1.12 0.18

The metal samples were machined to 19 mm×19 mm×2 mm coupons and polishedby hand through 1200 grit sandpaper then mechanically polished using0.05 micron alumina for up to 18 hours on a vibratory polisher.

A 10×10 square grid composed of 1 mm squares was laser machined onto thesurface using a pulsed ultraviolet laser in order to provide referencemarks for locating regions of interest to be tested.

The next step in the process for performing electrochemical measurementsof specific phase regions required identifying the different phases andregions of interest on the surface of the sample.

Optical microscopy provided sufficient contrast between the austenitephase and ferrite matrix on the surface and therefore was used for muchof the initial screening of the 2205 samples. However, in cases in whichthe crystalline orientation needed to be known, the screening processwas done from crystalline orientation maps obtained from electronbackscatter diffraction (EBSD) scans.

A next step in the procedure was the isolation of these selected phaseregions using the SMP technique.

This technique is based on laser lithography—a well-establisheddirect-patterning technique that avoids the complexity and cost of maskfabrication.

An UV (355 nm) solid state (Nd:YVO4) laser (Spectra-Physics YHP40) withmaximum pulse energies of ˜300 μJ and pulsewidths of ˜30 ns (FWHM) wasused as the laser source and the laser/motion-control system wasutilized.

A 1 μm-thick layer of Microposit S1818 photoresist was spin-coated ontothe sample surface and soft-baked at 110° C. for 1-2 min on a hot plate.

The areas of interest were then mapped by translating the sample on ahigh-resolution (10 nm) X-Y stage as it was exposed to individual 355 nmlaser pulses that were focused through a 20× microscope objective intoapproximately 40 μm diameter spots on the sample surface.

The energy of the laser spot was measured to be ˜2.6 μJ corresponding toa fluence of ˜200 mJ/cm² on the surface, which did not damage the steel,but was sufficient to expose the photoresist.

After exposure, the sample was immersed in a developer solution(Microposit MF-319) which removed the photoresist coating to reveal thebare steel surface of these selected regions while the rest of thesurface was still protected by the unexposed photoresist.

The sample was then hard-baked at 150° C. for 60 minutes to improve thecoating reliability and adhesion during corrosion testing.

Multiple ‘windows’ were opened on the surface coating and were robustenough to allow several corrosion experiments to be performed on thesame sample. FIG. 1 illustrates the laser lithography process on thesample.

Example 2

Electrochemical measurements were conducted.

Each photoresist-coated sample had multiple windows emplaced overvarious phase regions, so a procedure was developed to electricallyisolate the sample and the other windows so that only a single phaseregion or boundary was exposed to the electrolyte.

A small hole-punch (approximately 3 mm in diameter) was used to create awindow in the Kapton polymide tape that would isolate the window ofinterest from the surrounding grids. Kapton polymide tape was chosenbecause its adhesive properties did not damage the photoresist coatingbetween repeated application and removal.

A zip-lip, 2″×3″, 2 mil thick polyethylene bag was used to provideelectrical isolation from the electrolyte. A hole was cut into the bagusing a brass cutter that was larger than the Kapton tape window and theedges of the hole were sealed using Scotch Brand 1280 EMPelectroplater's tape.

The electrolyte used for all of the experiments was natural seawater.The seawater was pasteurized at 65° C. for a minimum of 12 hours priorto use. All of the electrochemical experiments were conducted in roomtemperature solutions.

Polarization scans that were performed in a deaerated environment wereobtained by bubbling argon gas into the sealed cell.

Example 3

The experiments were divided into two sets. The first set consisted ofproof-of-concept tests to determine the procedure that would be followedand to establish baseline measurements of the properties of thematerial. The second set of experiments was designed to explore theeffect of temperature on pitting corrosion in austenite and ferrite andto measure the characteristics of the oxide films that formed over theaustenite and ferrite using electrochemical impedance spectroscopy.

Example 4

Baseline Measurements.

For comparison to the individual grain experiments and to test thepreparation procedure for a 0.005 cm² portion of one of the grid cellswas exposed by the laser and tested in aerated seawater.

The current density from the exposed region ranged from 10⁻⁷ to ˜10⁻⁶A/cm² as the potential was increased from −0.5 to 1.0 V_(SCE).

Two other samples were selectively etched using a combination ofaggressive solution chemistries and potentiostatic holds at potentialsthat corresponded to the active corrosion peaks for each phase. Thisselective etching process removed either the ferrite or austenite grainsdepending on the potential.

These samples were then coated with the photoresist and lightly polishedusing 600 grit silicon carbide to remove the photoresist from theunetched phase.

Thus the electrochemical behavior of a high surface area of either theaustenite or ferrite could be obtained and compared to the response fromsamples prepared using SMP. These bulk exposed ferrite and austenitesamples were also tested in aerated seawater.

Example 5

Solution Resistance and Impedance Measurements.

As detailed in the following section, making electrochemicalmeasurements on such small areas of exposed metal presented severalchallenges to ensure that valid measurements were being obtained. Forexample, one of our concerns was that solution resistance, due tocurrent crowding in the vicinity of the exposed region of thephotoresist, would affect measurements.

To evaluate the magnitudes of the solution and polarization resistances,we performed impedance measurements on exposed austenite and ferriteregions that had been held at their open-circuit potential for two hoursin aerated seawater.

The solution resistance between the exposed region and the referenceelectrode for samples with areas on the order of 10⁻⁴ cm² was measuredto be 515 Ω=0.206 Ω·cm² from the Nyquist plots. An adjusted polarizationscan showing the corrected potential applied to the interface ascompared to the measured values was demonstrated.

The similarity between the two plots indicates that the ohmic IR dropbetween the sample and the reference electrode is negligible inpolarization scans done on the regions exposed by the micro-windows.

The polarization resistances varied widely for both austenite andferrite and independently of the exposed area. Possibly differences dueto crystalline orientation but these experiments demonstrated ability tomake the electrochemical measurement on SMP samples.

Example 6

Effect of the Size of the Exposed Area on the Current Measurements.

The area exposed on the sample following development of the photoresistwas determined using a numerical integration algorithm in the AliconaInfinite Focus Microscope image analysis routine.

For measurements conducted using a Gamry Series G 750 potentiostat,which can detect changes as small as 0.01 pA and has a leakage currentof around 5 pA, the experimental set-up had a low-current limit ofapproximately 100 pA—regardless of the type of exposed phase. Thistranslated into an exposed-area limit of no less than 3.0×10⁻⁴ cm². Formeasurements conducted using a Gamry Reference 600 potentiostat, withits correspondingly better sensitivity, the practical exposed area limitwas roughly 1.0×10⁻⁴ cm².

Once the size of the exposed area decreased, the potentiostat began tobe affected by the ambient electromagnetic noise in the room and waseventually unable to resolve the corrosion current throughout the entirepolarization scan, thus setting the lowest limit of exposed area formaking measurements.

Example 7

Austenite Phase Results.

In order to perform DC electrochemical experiments on individual phasesof known crystalline orientation, we mapped cells in the surface gridusing the EBSD camera to obtain the orientations of the various phasesand identify the grains to be tested. The phase regions that had similarorientations within a cell were then laser processed and developed priorto undergoing a polarization scan in deaerated seawater. Thepolarization scan from a test on a single region of the austenite phasewith a (111) orientation was demonstrated. Polarization scans from aselection of austenite phase regions with their respective crystallineorientations were also demonstrated.

Example 8

Ferrite Phase Results.

The polarization scan from a test on a single region of the ferritephase with a (203) orientation in deaerated seawater was demonstrated.Polarization scans from a selection of ferrite phase regions with theirrespective crystalline orientations were also demonstrated.

Example 9

Electrochemical Impedance Spectroscopy.

In this section, the impedance behavior of the ferrite and austenitephases in seawater is presented in contrast to the behavior of the bulkalloy in seawater. The impedance tests were carried out in 50 mVincrements from −400 mV_(SCE) to +1000 mV_(SCE). The AC potentials were±10 mV RMS and the frequencies ranged from 1×10⁵ to 1×10⁻² Hz at eachpotential. Nyquist plots for austenite, ferrite, and the bulk alloy at 5different potentials were demonstrated.

Assuming that the oxide film impedance can be modeled using a solutionresistance in series with a parallel circuit of a polarizationresistance and constant phase element, then the depth of theelectrically active region of the oxide film can be determined usingEquation 1.

$\begin{matrix}{C_{d\; 1} = {\frac{1}{Z_{CPE}} = \frac{{\epsilon\epsilon}_{0}A}{L}}} & 1\end{matrix}$

where C_(dl) is the double-layer capacitance, ϵ₀ is the free-spacepermittivity=8.85×10⁻¹⁴ F/cm, ϵ is the dielectric constant for the oxidefilm—assumed to be about 15.6, A is the exposed area, and L is the depthof the active region of the oxide film.

The measured currents of the polarization curves suggest that atpotentials below −400 mV_(SCE), the oxide film is removed from thesurface by the cathodic polarization but above −400 mV_(SCE) to about+200 mV_(SCE), the film covers the active surface and begins to thicken.Above +200 mV_(SCE), even though the surface is still passive, theelectro-active region of the film decreases, suggesting that the film isthinning or depleting a more active metal ion from the film. Above +400mV_(SCE), it appears as if another anodic reaction begins to dominatethe dissolution process, but above +850 mV_(SCE), that reaction isexhausted. Above +950 mV_(SCE), the current rapidly begins rising asoxygen evolution begins and the oxide film is dissolving as quickly asit is formed.

Example 10

Critical Pitting Temperature.

At room temperature in seawater, neither austenite nor ferrite in 2205is susceptible to pitting. The phases in these samples had estimatedPitting Resistance Equivalent Numbers (PREN) of 30.4 for the austeniteand 36.16 for the ferrite. However, as the temperature of the seawaterbath was elevated, metastable pitting events were observed above 45° C.with pitting occurring in ferrite around 61° C. and in austenite around63° C. These tests do show that SMP can be used to make Critical PittingTemperature measurements.

Using the SMP technique to mask off the untested areas of the 2205samples allowed polarization scans to be performed at slow scan ratesand without the concern of corrosion products obstructing the currentpath.

The high oven temperature and long, ˜60 minute baking times for thephotoresist inhibited the development of crevice corrosion under thephotoresist at the edges of the exposed regions.

The corrosion current was directly related to the dimensions of theexposed area of the sample so that it was possible to expose and developan area that had a corrosion current that was too low to be measured bythe available potentiostats.

Polarization resistance dominated the current path with values rangefrom 10⁶ ohms to 10⁹ ohms, with the solution resistance a manageablevalue of a few hundred ohms, as compared to solution resistances on theorder of 10³ ohms for a microcapillary of similar dimensions.

The relatively low solution resistance allowed us to perform impedancemeasurements on the individual phases.

Alloy 2205 is a passive alloy that does not pit at room temperature inseawater but its oxide film does break down at very anodic potentials.Both phases exhibited a passive current density on the order of 10⁻⁶A/cm² under anodic polarization between the open-circuit potential and asecondary dissolution peak around +0.5 V_(SCE), followed by anotherregion of passivity above the secondary dissolution peak before evolvingoxygen. However, as was shown above, as the temperature of the samplewas increased, stable pitting was observed in both phases. Plots weredemonstrated for the potential at which the corrosion current densityexceeded 10⁻⁴ A/cm² versus temperature for each phase and shows thatferrite and austenite behave quite differently at temperatures between60° C. and 65° C. This is consistent with earlier observations showingthat metastable pitting is different in austenite and ferrite andfurther demonstrates the ability to use SMP to obtain information onspecific grains.

Results obtained from using the SMP technique suggest there aredifferences in the behavior of the passive films as a function ofpotential and temperature, indicating that phase composition plays arole in the corrosion resistance of the alloy. Another step concerns thedifferences in corrosion behavior due to crystalline orientation andphase. For one example, polarization curves for austenite and ferritewith similar crystalline orientations were demonstrated.

The results obtained from using the SMP technique demonstrate that itcan be used to perform DC and AC electrochemical experiments on surfaceregions with an exposed area of <10⁻⁴ cm²—including irregularly-shapedregions such as cross-sectional areas of the dendrite fingers of theaustenite phase in the 2205 duplex stainless steel.

The area that can be interrogated is related to the current sensitivityof the potentiostat.

Although the application for this disclosure concerns corrosion, SMP haswide applicability to any electrochemical study that explores thebehavior of individual phase or isolated regions of interest.

Using SMP, differences in the impedance behavior of the oxide films andthe transition to stable pitting above the critical pitting temperaturehave been investigated for ferrite and austenite regions in a duplexstainless steel. The results also indicate crystalline orientationprobably does not play a role in the passive current density.

Some of the many advantages of this process over the previous ones areselection of the exact feature to be measured ahead of time and theability to conduct slow potentiodynamic tests without compromising thesample or losing signal information because of solution resistance.

Many modifications and variations of the present invention are possiblein light of the above teachings. It is therefore to be understood thatthe claimed invention may be practiced otherwise than as specificallydescribed. Any reference to claim elements in the singular, e.g., usingthe articles “a,” “an,” “the,” or “said” is not construed as limitingthe element to the singular.

What we claim is:
 1. An apparatus for electrochemical experimentationwith an isolated microstructural region on a surface comprising: a metalsample coated with a photoresist; a region of interest on the metalsample; a light source; wherein the light source comprises laser energyof about 2.6 microJoules corresponding to a fluence of about 200 mJ/cm²and is a 355 nm solid state laser with maximum pulse energies of about300 μJ and pulsewidths of about 30 ns and comprising optoelectronicdevices such as spatial light modulators or digital micromirror devicesfor direct modulation of the light distribution itself and avoiding theuse of a mask; an exposed region of photoresist and unexposed region ofphotoresist wherein the exposed region is created by light from thelight source and wherein the metal sample is immersed in a developersolution whereby removing the exposed region of photoresist and creatinga developed region of unexposed photoresist; and a water-resistantadhesive strip with a first perforated window over the region ofinterest and a sealed waterproof container with a second largerperforated window over the first perforated window.
 2. The apparatus forelectrochemical experimentation with an isolated microstructural regionon a surface of claim 1 wherein the regions of interest on the sampleare translated by an X-Y stage and exposed by light.
 3. The apparatusfor electrochemical experimentation with an isolated microstructuralregion on a surface of claim 1 utilizing a hard-baking step at atemperature of about 150° C. for about 60 minutes.
 4. A method forisolating microstructural regions or features on a surface forelectrochemical experimentation comprising the steps of: providing ametal sample; coating the metal sample with a photoresist; selecting aregion of interest on the metal sample; exposing the region of interestwith light and creating exposed photoresist and unexposed photoresistwith direct modulation of the light distribution itself byoptoelectronic devices such as spatial light modulators or digitalmicromirror devices and avoiding the use of a mask; and immersing themetal sample in a developer solution and removing the exposedphotoresist and creating a developed region of unexposed photoresist. 5.The method for isolating microstructural regions of claim 4 furtherincluding the step of revealing the bare metal surface of the region ofinterest while protecting the developed region of unexposed photoresist.6. The method for isolating microstructural regions of claim 5 whereinthe light can be pulsed or continuous and wherein the light is UV laserlight.
 7. The method for isolating microstructural regions of claim 6further including the steps of hard-baking the metal sample andimproving the coating reliability and adhesion and electricallyisolating the developed region of unexposed photoresist and furtherincluding the steps of polishing the metal sample prior to coating themetal sample with a photoresist and placing a grid reference mark on themetal sample prior to the step of selecting a region of interest of themetal sample, wherein the step of placing a grid reference mark is bylaser machining.
 8. The method for isolating microstructural regions ofclaim 7 further including the steps of placing a water-resistantadhesive strip with a first perforated window over the region ofinterest and sealing a waterproof container with a second largerperforated window over the first perforated window and conductingelectrochemical measurements, utilizing a 1 μm-thick layer ofphotoresist via spin-coating onto the metal sample, and furtherincluding the step of soft-baking at 110° C. for about 1-2 minutes on ahot plate.
 9. The method for isolating microstructural regions of claim8 wherein the light is UV laser light and utilizing a laser energy ofabout 2.6 μJ corresponding to a fluence of about 200 mJ/cm².
 10. Themethod for isolating microstructural regions of claim 8 utilizing a UVlaser light that is a 355 nm solid state laser with maximum pulseenergies of about 300 μJ and pulsewidths of about 30 ns.
 11. A methodfor isolating microstructural regions or features on a surface forelectrochemical experimentation comprising the steps of: providing ametal sample; coating the metal sample with a negative photoresist andthereby isolating the metal sample; selecting regions of interest on themetal sample; exposing the regions of interest with light energy andcreating regions of exposed negative photoresist and regions ofunexposed negative photoresist via utilizing a laser energy of about 2.6microJoules corresponding to a fluence of about 200 mJ/cm² and a UVlaser light that is a 355 nm solid state laser with maximum pulseenergies of about 300 μJ and pulsewidths of about 30 ns with directmodulation of the light distribution itself by optoelectronic devicessuch as spatial light modulators or digital micromirror devices andwithout a mask; immersing the metal sample in a developer solution andremoving the regions of unexposed negative photoresist and creatingdeveloped regions of remaining photoresist and revealing bare steelsurface of the regions of interest while protecting the regions ofexposed negative photoresist; and hard-baking the metal sample andimproving the coating reliability and adhesion and electricallyisolating the developed regions of remaining negative photoresist.